Sitting in front of an open fireplace in the 18th century, Benjamin Franklin noted, “The strongest heat from the fire, which is upwards, goes directly up the chimney and is lost.” And, honestly, Ben Franklin was right. An open, traditional fireplace is one of the most inefficient wood-burning heating appliances you can use to heat your home. In some cases, the lack of efficiency is so bad that an open fireplace can make your house colder. We at Santa’s Friend Chimney Service would like to tell you more about why the way open fireplaces work make them so inefficient.

Creating a draft is the most important mechanical function of an open fireplace.

Think about a hot air balloon. It rises because of a mass of hot air rises to its top and serves as its “motor.” This same concept goes on in a fireplace and chimney. The fireplace makes a column of heated gas inside the chimney. As that air goes up and exits out the chimney, more hot air from the fire is pulled after it. When this happens, a draft of smoke and hot gases up the chimney is generated. The draft also feeds necessary oxygen to the fire to keep it burning.

Knowing how heat moves can help you understand why an open fireplace can be so inefficient.

Heat moves by conduction, convection, and radiation. Conduction is when a hot object touches a cooler one, convection involves a movable substance, like hot air or liquid, circulating into cooler areas, and radiation is when warm electromagnetic waves, like sun rays, fire, and a heat lamp, carry heat to cooler objects to warm them by making their molecules move faster. A fireplace, of course, uses radiation to move heat. However, convection is also involved, and this is what makes an open fireplace so inefficient. Most of the heat created by a fire is in the form of hot gases. Convection pushes these gases out of the chimney and wastes them.

The draft also can lead to the inefficiency of an open fireplace.

The draft can actually draw more warm air from inside the house up the chimney, which makes the inside colder than it previously was. Experts estimate that open fireplaces can draw up to ten times as much air from the room than is needed to build the fire.

Open fireplaces can even have negative efficiency.

When more heat is lost by convection than is replaced by radiation, negative energy occurs. The colder the temperatures are outside, the colder the air is that gets sucked in by the fireplace. This means that the colder the outside air is, the less efficient your fireplace will be.